Sizing Conductors, Contactors, and Circuit Breakers for a Motor Circuit - Help

B

bindem

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Baton Rouge, LA
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Electrical Engineering Student
I’m working on a system design where a feeder circuit splits into two branch circuits, each supplying a motor:
  • One branch feeds a motor directly
  • The other feeds a VFD-driven motor
I’ve been going through the NEC but I’m having trouble pinpointing the relevant sections for conductor, breaker, overload, and contactor sizing.
Could anyone point me toward the right references or share how you typically approach this?
 
See NEC Art 430 Motors, Motor Circuits, and Controllers at first and then Art.210 and 215.
For conductor ampacity see Art. 310. For Overcurrent Protection see Art. 240.
 
Conductor ampacity for motor circuits may be different from Article 310, so there are sections in 430 addressing this too. Also, 430.122 is specific to the conductor size for feeding the line side of the VFD. Pretty much everything you need to know to meet NEC requirements for motor circuits is in Article 430.

Also, most VFDs have specific design requirements regarding the OCPDs ahead of them, with many now require specific fuses, even if you have a circuit breaker. The NEC only addresses this indirectly by requiring that you follow manufacturer instructions (110.3.B). So basically, RTFM… Any UL listed drive must have installation instructions that lay this out for you. If you are wanting to install a cheap Chinese internet VFD that is not UL listed, you are not compliant with the NEC anyway, so no point in asking.
 
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